1. Field of the Invention
The present invention relates to a high-speed tool steel gear cutting tool (hereunder referred to simply as a gear cutting tool) in which fractures or chipping (minute fractures) do not occur at the cutting edge even when performing gear cutting at high-speed, and which realizes excellent cutting performance over long periods, and a manufacturing method therefor.
2. Description of the Related Art
Heretofore, in the gear cutting of tooth profiles for various gears used as constituent members for automobiles, aircraft, and various drive units, a gear cutting tool such as for example the hob (solid hob) shown in FIG. 1, a pinion cutter, or a shaving cutter are used.
Furthermore, manufacture of this gear cutting tool by the following steps as illustrated hereunder in (a) to (e) it also well known.
(a) Hot forge an ingot of high-speed tool steel at a temperature of 1100 to 1150xc2x0 C. to make a bar with an outer diameter of 50 to 150 mm.
(b) Fully anneal the bar stock and then cut to a predetermined length, mill, and rough treatment into a tool material of a shape corresponding to the shape of the final gear cutting tool.
(c) Heat and hold the tool material at a temperature of 1210 to 1270xc2x0 C. in an atmosphere of nitrogen, and then quench the tool material by blow cooling with pressurized nitrogen gas, to transform the structure of the tool material into martensite.
(d) Heat and hold the post quenched tool material at a temperature of 500 to 550xc2x0 C. in an atmosphere of nitrogen to temper the tool material and transform the residual austenite dispersingly distributed in the matrix of the martensite structure formed in the quenching into martensite.
(e) Finish the post quenched and tempered tool material into a final shape by for example grinding or polishing.
Furthermore, as the abovementioned gear cutting tool, there is also known a coated gear cutting tool as disclosed for example in Japanese Patent Application, First Publication No. Hei7-310173, where the surface of the high-speed tool steel base metal is physical vapor deposited to an average thickness of 2 to 15 xcexcm with a hard coating layer comprising either or both of; a composite nitride [hereunder denoted by (Ti, Al) N] layer of Ti and Al and a composite carbonitride [hereunder denoted by (Ti, Al) CN] layer of Ti and Al, for which in the case where these are expressed by composition formula: [Ti1xe2x88x92xAlx]N and composition formula: [Ti1xe2x88x92xAlx]C1xe2x88x92mNm, the atomic ratio measured using an Auger spectroscopy analysis apparatus, of a central portion in the thickness direction satisfies x: 0.30 to 0.70, m: 0.6 to 0.99.
This coated gear cutting tool is shown for example in outline in FIG. 2, and is manufactured using a cathodic arc ion plating apparatus being one type of physical vapor deposition apparatus. In this case, for example the interior of the apparatus is made a vacuum atmosphere of 20 m torr, and in a condition heated with a heater to a temperature of 500xc2x0 C., an arc discharge is generated under conditions of for example voltage: 35 V, current: 90 A between an anode electrode and the cathode electrode on which is set a Tixcx9cAl alloy having a predetermined composition. At the same time, a nitrogen gas, or a nitrogen and methane gas, is introduced to inside the apparatus as the reaction gas, and a bias voltage of for example 200 V is applied to the base metal comprising the high-speed tool steel (hereunder simply base metal) so that the hard coating layer is physical vapor deposited on the surface of the base metal.
However, in recent times FA (factory automation) of gear cutting apparatus has become remarkable and there is a strong demand for labor saving and energy saving in the gear cutting process and a reduction in cost. Together with this, there is a demand for generality to enable a variety of gear cutting processes to be performed with only one type of gear cutting tool, and there is also a trend towards speeding up the gear cutting process.
As a result, in the conventional gear cutting tool, in the case where this is used in gear cutting under normal conditions there are no problems. However when used in high-speed gear cutting, this is susceptible chipping, particularly at the ridge line intersection of the rake face and the relief face of the cutting edge, so that the useful life is reached in a comparatively short time.
On the other hand, in the case of the conventional coated gear cutting tool, in the case where this is used in gear cutting under normal conditions, with carbon steel or cast iron or the like there are no problems. However when used in high-speed gear cutting of gears such as low-alloy steel or mild steel with extremely high viscosity, since the affinity between the chips produced by the cutting and the (Ti, Al) N layer or the (Ti, Al) CN layer constituting the hard coating layer is high, the chips are susceptible to adhering to the surface of the cutting edge of the gear cutting tool. This adhering phenomena becomes remarkably apparent the higher the speed of the gear cutting process, and with this adhering phenomena as the cause, fracture and chipping occurs at the cutting edge, so that the useful life is reached in a comparatively short time.
The present inventors, as a result of performing research from the abovementioned view point, into the manufacture of gear cutting tools for which the ridge line of the cutting edge demonstrates excellent anti-chipping even when used in high-speed gear cutting processes, have obtained the following research results shown in (1) to (4).
(1) In the case of the abovementioned conventional manufacturing method for a gear cutting tool, a 20 to 30 weight % (hereunder simply %) residual austenite exists in the martensite matrix in the tool material after quenching. Consequently, even if the residual austenite diffusingly distributed in the matrix of the martensite structure formed by the quenching is transformed into martensite, the existence of around 1 to 5% residual austenite cannot be avoided. This 1 to 5% residual austenite is comparatively coarse, and the shape thereof is nonuniform. Therefore this becomes a starting point for the chipping at the time of high-speed gear cutting.
(2) When the post quenched tool material is subjected to sub-zero treatment by cooling and holding at a temperature of below xe2x88x92150xc2x0 C., the residual austenite dispersingly distributed in a proportion of 20 to 30% in the matrix which has been transformed into martensite by the quenching is reduced to below 5%, and the shape thereof becomes fine and uniform.
(3) When tempering is performed on the post sub-zero treated tool material, a condition results where residual austenite is substantially non existent in the matrix which has been transformed into martensite, or exists but the proportion thereof is less than 0.5%. Moreover, the form thereof is extremely fine grained.
(4) In the gear cutting tool having a structure where residual austenite is substantially non existent in the matrix which has been transformed into martensite, or having a structure where residual austenite exists, but the proportion thereof is less than 0.5% and the form thereof is very fine grained, the starting point for chipping does not exist in the structure. Therefore, even if high-speed gear cutting is performed, there is no occurrence of chipping in the ridge line of the cutting edge, and excellent cutting performance can be demonstrated over a long period.
The present invention is based on the abovementioned research results, and is one where a method of manufacturing a gear cutting tool including: a step for quenching a tool material comprising high-speed tool steel and which has been rough processed to a shape corresponding to a final shape of a gear cutting tool, to transform a structure of the tool material into martensite, a step for tempering the tool material after quenching to transform any residual austenite dispersingly distributed throughout a matrix of the martensite structure formed by the quenching, into martensite, and a step for finishing the tool material after tempering to a final shape, is characterized in that the tool material after quenching is subjected to sub-zero treatment involving cooling and holding at a temperature of less than xe2x88x92150xc2x0 C., and transforming any residual austenite which is dispersingly distributed throughout the matrix into martensite, to thereby transform the structure of the tool material after tempering into a structure in which residual austenite which is a starting point for chipping at the time of high-speed gear cutting does not exist in the matrix of the martensite.
Furthermore, regarding the sub-zero treatment, preferably this is performed under conditions where the tool material after quenching is cooled using liquid nitrogen, at a predetermined cooling rate within a range of 1 to 10xc2x0 C./minute to a predetermined temperature within a range of xe2x88x92150 to xe2x88x92200xc2x0 C., and held at this temperature for a predetermined time within a range of 1 to 5 hours, and then raised in temperature at a predetermined temperature raising rate within a range of 1 to 10xc2x0 C./min.
The cooling rate, the cooling temperature of less than xe2x88x92150xc2x0 C., the holding time at the cooling temperature, and the temperature raising rate in the above mentioned sub-zero treatment are all empirically determined. In particular, regarding the cooling temperature, if the cooling temperature is higher than xe2x88x92150xc2x0 C., it is difficult to transform the residual austenite into the desirable martensite.
On the other hand, as a result of performing research into developing a coated gear cutting tool for which the adhesion of chips to the surface of the cutting edge is difficult, even in the case where this is used in a high-speed gear cutting process particularly for gears such as a low-alloy steel or a mild steel, the present inventors have obtained the following research results shown in (5) and (6).
(5) When Ta is dissolved in the (Ti, Al) N layer and the (Ti, Al) CN layer constituting the hard coating layer of the conventional coated gear cutting tool, so that the proportion held for the gross weight of Ti and Al becomes a proportion of 0.01 to 0.35 for the atomic ratio based on measurements using an Auger spectroscopy apparatus, of a thickness direction central portion, and a hard coating layer is made with the composite nitride of Ti, Al and Ta (hereunder shown as (Ti, Al, Ta) N) and the composite carbonitride (hereunder (Ti, Al, Ta) CN) layer of Ti, Al and Ta obtained from the results, due to the action of the Ta in this hard coating layer, the affinity to the work piece, in particular a highly viscous difficult to machine material such as low-alloy steel or mild steel is considerably reduced. Hence this has high chip lubrication properties. As a result, adhesion of the chips to the cutting edge is markedly suppressed. However, the high toughness held by the (Ti, Al) N layer and (Ti, Al) CN layer is lost.
(6) On the other hand, when the (Ti, Al) N layer and the (Ti, Al) CN layer constituting the hard coating layer of the conventional coated gear cutting tool, and the (Ti, Al, Ta) N layer and the (Ti, Al, Ta) CN layer shown in (5) above are alternately coated with the thickness of each extremely thin, that is an average thickness of 0.005 to 0.2 xcexcm, to form a hard coating layer, the problem points of the respective layers, that is the high affinity to the chips in the (Ti, Al) N layer and the (Ti, Al) CN layer (hereunder referred to as the first thin layer), and the low toughness in the (Ti, Al, Ta) N layer and the (Ti, Al, Ta) CN layer (hereunder referred to as the second thin layer) is cancelled out between the two, thereby furnishing the high toughness held by the first thin layer and the high chip lubrication properties held by the second thin layer. As a result, even when a coated gear cutting tool having this hard coating layer is used in high-speed gear cutting of a gear comprising a difficult to machine material of high viscosity such as a low-alloy steel or mild steel, chips are unlikely to adhere to the surface of the cutting edge, and excellent cutting performance is demonstrated over a long period.
The present invention is based on the abovementioned research results. The coated gear cutting tool is characterized in that a hard coating which is physical vapor deposited at an overall average thickness of 2 to 15 xcexcm, is formed on a surface of a base metal of a high-speed tool steel, by coating a first thin layer and a second thin layer with respective average thicknesses of 0.005 to 0.2 xcexcm,
the first thin layer comprising either or both of a (Ti, Al) N layer and a (Ti, Al) CN layer for which in the case where these are represented by
composition formula: [Ti1xe2x88x92xAlx]N 
and
composition formula: [Ti1xe2x88x92xAlx]C1xe2x88x92mNm, 
the atomic ratio based on measurements using an Auger spectroscopy apparatus, of a thickness direction central portion satisfies X: 0.30 to 0.70, m: 0.6 to 0.99, and
the second thin layer comprising either or both of a (Ti, Al, Ta) N layer and a (Ti, Al, Ta) CN layer for which in the case where these are represented by
composition formula: [Ti1xe2x88x92(X+Y)AlXTaY]N 
and
xe2x80x83composition formula: [Ti1xe2x88x92(X+Y)AlXTaY]C1xe2x88x92mNm,
the atomic ratio based on measurements using an Auger spectroscopy apparatus, of a thickness direction central portion satisfies X: 0.30 to 0.70, Y: 0.01 to 0.35 and m: 0.6 to 0.99.
In this coated gear cutting tool, the reason for making the average layer thickness of the first thin layer and the second thin layer constituting the hard coating layer respectively 0.005 to 0.2 xcexcm, is because if in either of these thin layers the average layer thickness becomes less than 0.005 xcexcm, the characteristics inherent in these thin layers, that is the high toughness inherent in the first thin layer and the high chip lubrication properties inherent in the second thin layer cannot be adequately imparted to the hard coating layer. On the other hand, if the average layer thickness thereof respectively exceeds 0.2 xcexcm, the inherent problem points of the respective thin layers, that is the chip adhering nature inherent in the first thin layer and the drop in toughness in the second thin layer, become apparent in the hard coating layer. The average layer thickness of the first thin layer and the second thin layer is more preferably 0.007 to 0.10 xcexcm for each.
Furthermore, in the coated gear cutting tool of the present invention, the Al in the (Ti, Al) N layer and the (Ti, Al) CN layer constituting the first thin layer of the hard coated layer, and in the (Ti, Al, Ta) N layer and the (Ti, Al, Ta) CN layer constituting the second thin layer, is added in order to improve the hardness with respect to TiCN, and improve the wear resistance. However, with the X value in
composition formula: [Ti1xe2x88x92xAlx]N and 
composition formula: [Ti1xe2x88x92xAlx]C1xe2x88x92mNm, and also in 
composition formula: [Ti1xe2x88x92(X+Y)AlXTaY]N and 
composition formula: [Ti1xe2x88x92(X+Y)AlXTaY]C1xe2x88x92mNm, 
is less than 0.30 for the atomic ratio (and similarly hereunder), the desired wear resistance cannot be ensured. On the other hand, if the above value exceeds 0.70, then fracture or chipping of the cutting edge is likely to occur. Therefore, in the present embodiment, the X value is set at 0.30 to 0.70. This X value is more preferably 0.35 to 0.65.
Furthermore, since the C component in the abovementioned (Ti, Al) CN layer and the (Ti, Al, Ta) CN layer provides an affect of improving the hardness, then the (Ti, Al) CN layer and the (Ti, Al, Ta) CN layer each have a relatively high hardness compared to the abovementioned (Ti, Al) N layer and the (Ti, Al, Ta) N layer. However, if the proportion of the C component in the above mentioned composition formula is less than 0.01, that is the m value exceeds 0.99, the predetermined hardness improvement effect is not obtained. On the other hand, if the proportion of the C component exceeds 0.4, that is the m value is less than 0.6, the toughness decreases suddenly. Therefore, in the present invention, the m value is set at 0.6 to 0.99. This m value is more preferably 0.8 to 0.9.
Furthermore, due to the effect of the (Ti, Al, Ta) N layer and the (Ti, Al, Ta) CN layer constituting the second thin layer, this has excellent chip lubrication compared to the (Ti, Al) N layer and the (Ti, Al) CN layer of the first thin layer. However, if the Y value in the abovementioned composition formula is less than 0.01, the Ta content becomes insufficient, so that the predetermined chip lubrication improvement effect cannot be imparted to the hard coating layer. On the other hand, if the Y value exceeds 0.35, the toughness decreases rapidly in the overall hard coating layer. Therefore, in the present invention, the Y value is set at 0.01 to 0.35. This Y value is more preferably 0.07 to 0.30.
Furthermore, the reason for making the overall average thickness of the hard coating layer 2 to 15 xcexcm, is that at a layer thickness of 2 xcexcm a desirable excellent wear resistance cannot be ensured, while if the layer thickness exceeds 15 xcexcm, fracture or chipping of the cutting edge is likely to occur. The layer thickness is more preferably 3 to 10 xcexcm.